Check in at exactly 24 hours before departure, and you even have a reasonable chance of getting a seat in the front/bulkhead row (which some find more spacious, and are often blocked for passengers with status).
If I didn't have BA status, I *might* think about paying for seat selection on long-haul, as some seats really are better than others, but I wouldn't bother with a short-haul flight. Seats are almost all the same, and a LHR-CDG segment is very short regardless.